1. Field of the Invention
The present invention relates generally to optical devices and, in particular, to a lamination-based method for the micro-fabrication of optical assemblies and other small components, and to a temperature compensator for a Faraday rotator.
2. Background of the Related Art
When light rays emitted from a light source are transmitted through an optical system, part of the light rays will be reflected at the end face of the optical system and transmitted back to the light source, unless means are employed to prevent such back reflection. For instance, in transmitting an optical signal through an optical fiber, if a light beam emitted from a laser light source is projected onto the end face of the optical fiber through, for example, a lens, the majority of the light thereof will be transmitted through the optical fiber as transmitted light beam. But, a part of the light thereof will be surface reflected at the end faces of the lens and the optical fiber and transmitted back to the laser light source. This back reflected light will again be reflected at the surface of the laser light source, thereby creating undesirable reflection-induced noise.
To eliminate such noise, an optical isolator, as described for instance in Bellcore's Special Report, Optical Isolators: Reliability Issues, SR-NWT-002855, Issue 1, December 1993, Pages 1-3, incorporated herein by reference, may be used. This is an example of an optical device that allows light to propagate (with relatively low loss) in one direction but isolates reflected light from propagating in the reverse direction. Optical isolators are used to improve the performance of many devices such as external modulators, distributed feedback lasers, Fabry-Perot lasers, semiconductor amplifiers, and diode-pumped solid-state lasers among others.
Optical isolators are typically passive, non-reciprocal optical devices based on the Faraday effect. In 1842, Michael Faraday discovered that the plane of polarized light rotates while transmitting through glass which is contained in a magnetic field. The Faraday effect is non-reciprocal, meaning that the direction of rotation is independent of the direction of light propagation, and only dependent upon the direction of the magnetic field. Most commercial optical isolators utilize this effect to isolate various parts of an optical communication system from reflection-induced noise.
Typically, an optical isolator consists of a magneto-optical material called a Faraday rotator which is sandwiched between a pair of polarization elements commonly referred to as a polarizer and an analyzer. The Faraday rotator is used in optical devices, such as the optical isolator, to rotate the plane of polarization that is incident upon it by a predetermined amount, usually by 45.degree. either clockwise or counter clockwise. Typically, the Faraday rotator is a garnet crystalline structure with an inherent magnetic field, so that the direction of Faraday rotation is predetermined. In some cases an external magnetic field may be needed to activate the Faraday rotator. In such cases, the direction of Faraday rotation is dependent on the orientation of the magnetic field but not on the direction of light propagation. As used in the telecommunication industry, the Faraday rotator is essential to many devices that utilize its properties in combination with reciprocal polarization elements.
In the pass (forward) direction, light incident on the polarizer will pass through the polarizer without obstruction if its plane of polarization coincides with that of the polarizer. When this light passes through the Faraday rotator its plane of polarization is rotated by 45.degree. due to the magneto-optic effect. The direction of rotation, that is, clockwise or counter clockwise, is dependent on the particular Faraday rotator configuration and is predetermined. The light then passes through the analyzer without loss, since the axis of polarization of the analyzer is oriented at the same 45.degree..
In the blocking (reverse) direction, reflected light of arbitrary polarization is incident on the analyzer which transmits some of this light and polarizes it to match its axis of polarization. When this polarized reflected light passes through the Faraday rotator its plane of polarization is again rotated by 45.degree., clockwise or counterclockwise relative to the direction of light travel, as is predetermined. As a result, the plane of polarization of the reflected light incident on the polarizer is perpendicular to its axis of polarization, and, thus the reflected light is blocked by the polarizer. In this manner, the optical isolator is used to transmit light from a source in the pass (forward) direction and essentially extinguish any reflected light in the blocking (reverse) direction. This extinguishing effect is commonly known as "isolation".
The magnitude of the rotation of the plane of polarization of light passing through the Faraday rotator depends on several factors, such as, the strength of the magnetic field, the nature of the material that constitutes the rotator, the frequency of the light, the temperature, and other parameters. Since the components in many optical applications utilizing the Faraday effect may be exposed to temperature variations, the rotational temperature dependency of the Faraday rotator limits the use of Faraday rotators in devices which do not provide some form of temperature compensation to prevent or minimize degradation in performance.
The rotational temperature dependency of a Faraday rotator can be expressed in terms of a temperature coefficient of rotation, C.sub.ROT, defined as: ##EQU1##
where, .theta. is the rotation of the plane of polarized light passing through the Faraday rotator, and T is the temperature. A typical Faraday rotator may have a temperature coefficient with a magnitude of as much as about 0.10.degree./.degree. C. which can cause a variation of Faraday rotation of about 12.degree. over a temperature range of about -40.degree. C. to 85.degree. C. Of course, such undesirable rotation of the light can have significant detrimental effects on the performance of an optical device both in terms of forward transmissivity and degree of reverse isolation. But, since isolation (attenuation in the blocking direction) is measured very close to zero, small changes can have orders of magnitude effects on the degree of isolation in terms of the blocking direction transmission of reflected light.
One proposed solution to this problem is to provide temperature compensation via a cooling/heating source which maintains the temperature of the Faraday rotator, and possibly the temperature of the entire device, including for example, the laser source, at the required value. This would require that the temperature of the Faraday rotator be monitored and the output from the cooling/heating source be adjusted accordingly. Thus, the components required in such a temperature compensation system would include a cooling/heating source, temperature measurement device, a feedback system, and a power supply among others. Disadvantageously, such a temperature compensation scheme not only adds to the complexity and cost of the device, but also to the size of the optical device which can limit the use of the device in many applications.
In some cases, a cascaded isolator, such as a double stage isolator, is utilized to compensate for the effects of temperature variance on optical isolators. Typically, a double stage isolator utilizes a polarizer, a Faraday rotator, an analyzer/polarizer, a second Faraday rotator, and a second analyzer arranged in this sequence. This effectively provides two stages of optical isolators in series. Typically, to compensate for temperature variations, one stage is "de-tuned" to an offset temperature above the ambient temperature while the other stage is "de-tuned" to an offset temperature correspondingly below the ambient temperature so as to provide a more broad-band response between the two temperature extremes. However, such detuning results in overall degraded isolation performance over the temperature range and at the nominal design temperature. Another proposed solution is to cascade multiple stages of isolators. But, the use of cascaded isolators in an optical device, undesirably, not only adds to the complexity, cost and size of the device, but, also increases the number of components needed, and increases the optical path of the light while reducing overall transmissivity through the cascaded isolators.
Optical components, such as the polarizer, the Faraday rotator, and the analyzer of a typical optical compensator, are commonly fixed in an assembly or attached to a common substrate. The primary approach, in the industry to date, to fixturing optical components involves the implementation of screw-machined barrels or small blocks with counter-bored features machined in. The optical components are placed in these machined cavities which typically tend to be small in size (for example, less than 2 mm in diameter). Not only is the machining process of generally tiny metal fixtures a costly and time consuming operation, but, also the discrete approach of fixing the optical components is generally not suited for mass automation. Undesirably, such a method of fixturing optical components is labor intensive and leads to higher manufacturing costs and lower manufacturing efficiency.
Thus, there is a need for providing a Faraday rotator temperature compensator that is simple, low cost and dimensionally small and there is a need to provide an efficient and low cost method that is well adapted for the automated manufacturing of such optical assemblies and other small components.